MongoDB Aggregate pipeline with $group and $count on a Pointer reference returns wrong data - database

Question

I have a set of data in MongoDB with parse-server in the following format-
Rating => objectId, user<_User>, rating...
_User => objectId, gender<m|f|nb|na>
I have been trying to group the data based on the user's gender to find out how many male, female, non-binary or N/A users have rated. user field in a pointer reference to _User. I am using the following aggregate pipeline.
const pipeline = [
{
lookup: {
from: '_User',
localField: 'user',
foreignField: 'objectId',
as: 'user'
}
},
{
unwind: { path: '$user' }
},
{
group: {
objectId: '$user.gender',
count: {
$sum: 1
}
}
}
]
const data = await new Query('Rating').aggregate(pipeline)
Result =>
[
{
"count": 54,
"objectId": "na"
},
{
"count": 405,
"objectId": null
},
{
"count": 27,
"objectId": "f"
},
{
"count": 540,
"objectId": "m"
}
],
However, returned data count doesn't match with actual data. The actual database has only 27 ratings with 1 f, 2 na, 24 m.
For MongoDB developers, objectId is equavalent to _id.
I am a novice to aggregation framework. What am I doing wrong?
Server Environment-
parse-server: 3.2.3
mongodb: 4.0.2

Answer 1

It is tricky because you need to understand how Parse Server stores the data inside the MongoDB. The following query should solve your problem:
const query = new Parse.Query('Rating');
const pipeline = [
{
project: {
objectId: 1,
userId: { $substr: ['$_p_user', '_User$'.length, -1] }
}
},
{
lookup: {
from: '_User',
localField: 'userId',
foreignField: '_id',
as: 'user'
}
},
{
unwind: { path: '$user' }
},
{
group: {
objectId: '$user.gender',
count: {
$sum: 1
}
}
}
];
return await query.aggregate(pipeline, { useMasterKey: true });